Dielectric compositions



Nov. 22, 1960 Dielectric Constant G. GOODMAN 2,961,327

DIELECTRIC COMPOSITIONS Filed Oct. 50, 1958 Fig.2.

k E 1 5 Q /00- 22 2/ i I I00 200 300 15a 250 Temperature C Temperature"C Dielectric Constant 'k 4 8 6 o o Q Q 1 s I i i l i JV /3 I00 200 300Temperature "6 Fig.3.

s00 /nvent0r:

l so o by -7. Lvwh His Attarne y.

United States Patent .0

DIELECTRIC COMPOSITIONS Gilbert Goodman, Schenectady, N.Y., assignor toGeneral Electric Company, acorporation of New York Filed Oct. 30, 1958,Ser. No. 770,726 4 Claims. (c1. 106 -39) This invention relates todielectric compositions and more particularly to dielectric compositionsdisplaying ferroelectric characteristics which within their usefultemperature range are not subject to a Curie temperature at whichtransition to paraelectric behavior occurs, and further relates tobodies made of such dielectric compositions.

The known ferroelectric substances can be divided generally into twogroups: (1) substances like Rochelle salt, KH PO and guanadine aluminumsulphate hexahydrate and their isomorphs; and (2) inorganic oxidecompounds. The members of the first group are subject to severelimitations in use due to certain basic difficulties having to do withCurie temperatures located close to room temperature, and with theirsolubility in water.

Barium titanate, BaTiO is the prototype and most exploited member of thesecond group, and it is most commonly used in the form of ceramics,although in applications such as computer memory circuits, singlecrystals are utilized. Barium titanate is subject to a Curie temperatureof about 125 C., where the ferroelectric properties transform toparaelectric properties. Obviously, when either a barium titanateceramic body or a barium titanate single crystal must be used atelevated temperatures, as they would be in some electrical andelectronic apparatus, they can no longer be depended upon to have thedesired ferroelectric characteristics.

It is therefore a principal object of this invention to provide adielectric composition which has no observable Curie point up to thepoint where conduction becomes dominant at an elevated temperature andthe material is no longer useful as a dielectric.

'It is another object of this invention to provide ceramic or singlecrystal dielectric bodies which can be used at ele-- vated temperaturesand still possess ferroelectric properties.

Other objects and advantages of this invention will be in part obviousand in part explained by reference to the accompanying specification anddrawings.

In the drawings:

Fig. l is a graph showing the effect of temperature on the dielectricconstants of various compositions made according to the presentinvention;

Fig. 2 is a graph showing the elfect of temperature and compositionalvariations on the dielectric constants of additional compositions;

Fig. 3 is a graph which also shows the relationship of compositional andtemperature variations onthe dielectric constants of the compositions.

Generally, the present invention relates to dielectric compositions andto bodies made of dielectric compositions which exhibit no Curie pointup to the temperature More-specifically, the compositions of the presentin I Vntion are preferably composed essentially of a vitrified or fusedcombination of (a) barium oxide, (b) strontium oxide, and (c) niobiumoxide, the combined molar amounts of strontium and niobium metals inoxide form being present in predetermined amounts with respect to themolar quantity of niobium metal present as niobium When preparing amixture of the selected components, it will be appreciated they can becombined as metals and subsequently oxidized, or, as is done in mostinstances,

the materials can be combined as oxides since they are normally morereadily and inexpensively obtainable in this condition. They may also beadded as oxygen-containing compounds, such as barium and strontiumoxalates and/or carbonates noted in the following Table I, as early inthe sintering or fusing process the oxalates or carbonates will breakdown to form the oxide of the metal, which then becomes part of thefinal product.

The following Table I lists the nominal weight percent composition ofpolycrystalline sample, made according to the present invention.

Table I Composition-Weight Percent Sample No. Firing Temp.

and Time Ba- 81-00;" Nbz05 Oxalate 20.03 20. 71 59. 25 1,359" 0., 3 hrs.26. 05 16.17 57. 76 1,350" 0., 3 hrs. 31. 78 11.82 56. 37 1,400 0., 3hrs. 37. 23 7. 70 55. 07 1,425 0.. 3 hrs. 13. 71 25. 50 ,60. 77 l,4000., 3 hrs. 36.63 22. 74 40. l,350 0., 1 hr. 51.39 10.62 37.99 1,359 0.,1 hr. 19. 68 36. 63 43. 68 1,350 0., 1 hr. 45. 60 9. 45 44. 1,350" 0., 3hrs. 32. 28 20.02 '47. 70 1,350 0., 1 hr. 17.19 31.98 50.81 1,350 0., 1hr.

* Assaying 64.0% 13210. Assaying 69.72% 810.

Ceramic bodies having thepreceding compositions were made by mixing thequantities of the selected components listed in Table I in a ball millin amyl acetate for between 5 and 10 hours, the exact time for mixingnot being critical as long as retained within the stated limits. Alcohol can be used in place of am'yl acetate if desired. Following millingof the materials, they were dried and calcined in a platinum containerat about 1200 C. for 3 hours, crushed to -200 mesh, because of theagglomeration which accompanied calcination, and Carbowax was added toachieve consistency suitable. for pressing the material into one inchdiameter buttons, A; inch thick. The buttons were then fired in air on aplatinum base at the temperatures and for the times indicated in TableI. Neither the temperature nor the time is particularly critical, as thetime at maximum temperature may vary as much as one hour and the maximumtemperature as much as 25 C., for example.

Ceramic bodies prepared as outlined above are polycrystalline structurescontaining primarily a ferroelectric crystal phase responsible for theunique properties, and some additional material which may be considereddross. By removing the single crystals from the ceramic, bodies areobtained which have maximum dielectric values. On the other hand, formanyapplications the properties possessed by polycrystalline bodies aresuitable, since they are qualitatively similar to the properties of asingle crystal. I i

When single crystals are the primary objective, larger andvmore suitablesamples are obtained by melting and recrystallizing the batchconstituents'than from sintering asdescribed' previously. For example,if mixed con; stituents of composition #10 of Table I are heated in aPatented Nov. 22, 1960- platinum crucible to 1600 C., cooled at a rateof C. per hour to 1400 C., and then cooled more rapidly to roomtemperature, a solidified mass of crystals is obtained from which suchindividual crystals can be easily extracted.

Referring to the drawings to illustrate the properties of dielectricbodies made according to the present invention, the curves are numberedto correspond with the compositions shown in Table I. In Fig. 1 thecurves indicate the effect of temperature on the dielectric constant ofceramics in which the total amount of barium and strontium present inthe oxides is equal essentially to half the amount of niobium metalpresent as niobium oxide. While the total combined amount of barium andstrontium is about one-half that of the niobium, the amounts of bariumand strontium are varied so that the ratio between the two metals isdifferent for each of the samples used to obtain the four curves. Thisprocedure was adopted to determine the effect of compositionalvariations on the dielectric constants at different temperatures.

The curves of Fig. 1, specifically 10-1'3, indicate that the dielectricconstant passes through a rounded maximum and then decreases at aboutthe same rate as the increase initially occurred. In prior ferroelectricmaterials, dielectric constant maxima or peaks corresponding to those ofthe curves in Fig. 1, normally represent the Curie point, where existingferroelectric properties convert to paraelectric properties. This is notthe case with the present compositions since they continue to exhibitferroelectric properties, such as hysteresis beyond the region ofmaximum dielectric constant. The maxima in the curves are associatedwith a dielectric relaxation.

Referring specifically to curve =10, the barium metal, which as alreadymentioned, is present as the oxide in the completed ceramic, is presentin an amount such that there is a ratio of 0.375 mol of barium to 0.625mol of strontium metal, the latter also being present in oxide form.Thus, the combined amount of barium and strontium is equal to 1 mol andthe niobium metal is present in an amount of 2 mols. In curve 11 theratio has been changed so that the amount of barium has been increasedto 0.5 and the strontium decreased to 0.5. Similarly, in curves 12 and13 the barium is increased to 0.625 and 0.75 mol respectively and thestrontium decreased to 0.375 and 0.25 mol, respectively. Two mols ofniobium are present in all compositions.

The composition whose properties are represented by curve 14 of Fig. 1contains 0.25 mol of barium, 0.75 mol of strontium and 2 mols ofniobium. In this instance, the maximum has apparently been shifted tosuch a low temperature that the maximum is not shown.

Comparing all of the curves, it will be seen that the relative increasein the amount of barium and decrease in the amount of strontium resultsin a lowering of the maximum dielectric constant and a shifting of themaximum point to higher temperatures. This eifect can, of course, beutilized where it is desired to have a dielectric material which has arelatively high dielectric constant at elevated temperatures. Thedielectric constant measurements for curve 10 where taken at a frequencyof 1 megacycle per second whereas the dielectric constant measurementsfor curves 11-14 were made at l kilocycle per second.

Although single crystal bodies have the highest dielectric constants andordinary polycrystalline bodies have the lowest ones, it is possible toobtain polycrystalline masses which are composed largely of singlecrystals arranged in a common orientation. Thus, for example, if barium,strontium, and niobium oxides are mixed in the mol ratios of 0.5 to 0.5to 1, respectively, and lowered slowly in an elongated platinum cruciblethrough a tube furnace whose hot zone is above the melting point of themixture a boule, or ceramic body, having an oriented microstructure isobtained. In much of this boule, the structure consists of about byvolume of crystals having common axes aligned in parallel relationship.As a result of this orientation the boule possesses dielectricproperties intermediate between those of a single crystal and those of aconventional ceramic.

An important factor to be considered, in order that the ternarycomposition have optimum dielectric and ferro-electric properties, isthe molar ratio existing between the combined amounts of barium andstrontium metals present and the amount of niobium metal present.Specifically, the total molar concentration of the (a) and (b) groupmetals (barium and strontium) preferably should equal approximatelyone-half the amount of niobium metal present in its oxide. That is, ifthere are 2 mols of niobium present, then the sum of the barium andstrontium metals should be 1 mol so that substantially 0.5 to 1 molarratio is established.

Generally, in order for optimum properties to be achieved, thedielectric body should contain, for each 2 mols of niobium present asniobium oxide, from 0.25 to 0.75 mol parts or barium as barium oxide andfrom 0.25 to 0.75 mol parts of strontium as strontium oxide. The totalcombined amounts of barium plus strontium present may vary from 0.75 molto not more than about 1.50 mols, amounts on either side of this rangereducing the results to the point where they are unacceptable for normaldielectric purposes.

Referring to Fig. 2 of the drawings, curves 20, 21 and 22 illustrate theeifects of varying the ratio between the total amount of barium andstrontium and the total amount of niobium, as well as the effect ofchanging the ratio between the barium and strontium. In all cases,however, there are two mols of barium and strontium for each two mols ofniobium, this resulting in a 1 to 1 mol ratio. The curve 20 shows theeifect of temperature on the dielectric constant of a ceramic body madeaccording to the previously described process and containing metals inthe mol ratios of 1.0Ba:1.0Sr:2.0Nb. It will be noted that thedielectric constant apparently increases slightly up to about C., atwhich time it appears to increase rap idly toward a maximum value.Actually, the upward trend of the curve 20 results from a conductionetfect indicating deterioration of dielectric properties. Ultimately thedeterioration proceeds to the point that the material is no longeruseful as a dielectric.

Dielectric values essentially similar to those indicated by curve 20 areshown by curves 21 and 22. Here, the dielectric constants aresubstantially invariant up to temperatures around C., where after therebegins the apparent rise from a conduction elfect. These dielectricshave metal mol ratios of 1.5Ba:0.5Sr:2.0Nb, and 0.5Ba:1.5Sr:2.0Nb,respectively.

*Curves 25 through 27 of Fig. 3 were obtained by using ceramic bodies inwhich the combined amount of barium and strontium provided 1.5 mols ofthese materials for each 2 mols of niobium. In these ceramics the molproportions of barium and strontium are: 0.37 5Ba:1.l25Sr:0.75Ba:0.175Sr; and 1.125Ba:O.375Sr for curves 25, 26 and 27respectively. These ceramics have dielectric constants which peak atfairly low temperatures, although ferroelectric properties are retainedup to elevated temperatures where the conduction effect begins to takeplace.

Thus, this invention provides dielectric compositions which can be usedfor high temperature applications Where ferroelectric properties aredesired and where relatively high dielectric constants are either neededor preferred.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A dielectric body consisting essentially of a combination of (a)barium oxide, (b) strontium oxide, and niobium oxide, the barium andstrontium metals of the oxides of groups (a) and (b) being present insuch molar ratio that there are from 0.25 to 0.75 mol of barium presentper 0.75 to 0.25 mol of strontium, the total molar concentration ofbarium and strontium being equal to about 1 and being equal essentiallyto /2 the molar concentration of niobium in said niobium oxide.

2. A dielectric body consisting essentially of a combination of (a)barium oxide, (b) strontium oxide, and (c) niobium oxide, barium andstrontium metals of the oxides of group (a) and (b) being present insuch molar ratios that they are from 0.375 to 0.625 mol of bariumpresent per 0.625 to 0.375 mol of strontium, the total molarconcentration of barium and strontium being equal to about 1 and beingequal essentially to /2 the molar concentration of niobium in saidniobium oxide.

3. A dielectric body as defined in claim 2 wherein 5 of barium andstrontium present ranging from not less than about 0.75 to not more thanabout 1.50 mols.

References Cited in the file of this patent UNITED STATES PATENTSGoodman Sept. 3, 1957 OTHER REFERENCES J. Amer. Ceramic Soc., vol. 371954), pages 581-588. Pchelkin et al.: J. Gen. Chem, U.S.S.R., vol. 24

said barium and said strontium are present in 0.5 mol 15 (1954), pages1284-1286. (Chem. Abstracts, vol. 49,

quantities.

4. A DIELECTRIC BODY CONSISTING ESSENTIALLY OF (A) 0.25 TO 0.75 MOLPARTS OF BARIUM AS BARIUM OXIDE, (B) 0.25 TO 0.75 MOL PARTS OF STRONTIUMAS STRONTIUM OXIDE, AND (C) 2 MOL PARTS OF NIOBIUM AS NIOBIUM OXIDE, THETOTAL AMOUNT OF BARIUM AND STRONTIUM PRESENT RANGING FROM NOT LESS THANABOUT 0.75 TO NOT MORE THAN ABOUT
 1. 50 MOLS.